Interest-Only Loan Calculator: Use the Formula
The first time you run an interest-only loan calculator, the number looks almost too good to be true. That low monthly figure comes with a catch: you never chip away at the money you borrowed. This article breaks down the formula, works through a real £200,000 mortgage example, and walks you through the lender checks that decide whether you can actually get one.
Monthly payment on £200,000 at 4% interest-only: £666.67 · Monthly payment on £200,000 at 5% interest-only: £833.33 · Monthly payment on £200,000 at 6% interest-only: £1,000 · Principal reduction during interest-only period: £0 unless the borrower overpays
Quick snapshot
- Monthly interest-only payment = loan × annual rate ÷ 12 (Charles Cameron & Associates, UK mortgage broker)
- No principal is repaid during the interest-only term unless the borrower makes overpayments (Charles Cameron & Associates, UK mortgage broker)
- UK lenders must verify a credible repayment strategy for interest-only mortgages (Financial Conduct Authority, UK regulator)
- Which UK lenders are currently offering interest-only products and the minimum equity they demand
- How a specific lender assesses investment returns as part of the repayment strategy check
- Whether an interest-only loan is a good idea depends entirely on the borrower’s cash flow, investment returns, and risk tolerance
- Application: income, credit, valuation, and repayment strategy assessed
- During interest-only period: only interest is paid; balance stays the same
- End of term: full capital due; refinance, sell, or use a lump sum
- After term: must repay or negotiate a new mortgage product
- See the formula and a £200,000 worked example below
- Compare interest-only vs repayment on monthly cost and total cost
- Check whether you meet current lender availability and approval requirements
Key facts at a glance
The table below distils the numbers that matter most for any interest-only borrower.
| Variable | Value |
|---|---|
| Payment formula | Loan amount × annual interest rate ÷ 12 |
| £200,000 at 5% | £833.33 a month |
| £200,000 at 6% | £1,000 a month |
| Principal during interest-only | Unchanged |
| Balance at the end of term | Still £200,000 if no overpayments |
How are interest-only loans calculated?
The interest-only payment formula
The formula is straightforward: multiply the loan amount by the annual interest rate, then divide by 12. For example, borrowing £200,000 at 5% gives you £200,000 × 0.05 ÷ 12 = £833.33 per month. This calculation is used by major UK lenders including Halifax (one of the UK’s largest mortgage lenders) and Lloyds Bank (major UK high-street bank). The Bank of England (UK central bank) also provides a borrowing calculator that breaks down the monthly interest paid.
Calculator inputs and sample output
- Loan amount (e.g., £200,000)
- Annual interest rate (e.g., 5%)
- Loan term (does not affect the interest-only payment, only the deadline)
A UKCalculator (free online calculator site) example shows that £200,000 borrowed at 4.5% gives a monthly interest-only payment of £750 — derived as £200,000 × 4.5% ÷ 12.
Why the principal balance stays the same
Because you pay only the interest each month, the original loan amount does not decrease. As Charles Cameron & Associates (UK mortgage broker) explains, the borrower makes no capital repayments unless arranged separately. That means the full £200,000 is still due at the end of the term.
A buyer who borrows £200,000 at 5% for 25 years and makes only the interest payment will have paid £250,000 in interest alone by the end — and still owe the original £200,000. Every pound that goes to interest is a pound that does not build equity.
The implication: A borrower who treats the low monthly payment as permanent risks a stark financial reversal at term end, when the full £200,000 falls due and no equity has accumulated.
How much will I pay in interest-only payments on a mortgage worth £200,000?
Worked example: £200,000 at different rates
Four rates, four monthly payments — same outstanding balance at the end.
The table below shows how a seemingly small rate move compounds into thousands in extra interest.
| Annual interest rate | Monthly interest-only payment | Total interest over 25 years (no overpayments) |
|---|---|---|
| 4% | £666.67 | £200,000 |
| 5% | £833.33 | £250,000 |
| 6% | £1,000 | £300,000 |
| 3% | £500 | £150,000 |
The 3% row comes from a Charles Cameron & Associates broker calculator that shows £500 a month for a £200,000 mortgage at 3%.
UK mortgage rates and interest-only payment
Current UK interest-only rates fluctuate with the Bank of England base rate. The MoneySavingExpert (leading UK consumer-finance website) mortgage rate calculator lets you toggle between repayment and interest-only to see the difference in real time. Their calculator uses the same formula but factors in product fees to show total cost.
The implication: your monthly interest-only bill rises and falls with the Bank of England rate. A 1% increase on a £200,000 loan adds £166.67 to your monthly payment — no warning, no grace period.
What this means: A borrower on a £200,000 interest-only mortgage faces a £166.67 monthly payment swing for every 1% rate move, with the full £200,000 balance still due at the end.
Can you still get an interest-only loan?
Is it hard to get approved for an interest-only loan?
Interest-only mortgages are still available from many lenders, but underwriting is significantly stricter than for repayment mortgages. HSBC UK (global bank with large UK mortgage book) offers a calculator specifically for temporary interest-only arrangements, not for new full-term interest-only borrowing. Lenders such as Teito (UK mortgage broker) report that you can generally borrow up to 75% loan-to-value on an interest-only basis — meaning you need at least a 25% deposit.
Do banks still do interest-only loans?
Yes, but the market has shrunk since the Financial Conduct Authority tightened rules in 2014. The FCA (UK financial regulator) now requires lenders to assess whether the borrower has a credible repayment strategy — for example, proceeds from selling the property, an investment portfolio, or a pension lump sum. Without that, approval is unlikely.
Can you get a 0% interest loan?
0% interest loans are not a standard mortgage category. They occasionally appear in promotional consumer credit (e.g., 0% on a new sofa) but not on secured loans as large as a mortgage. Any offer claiming “0% mortgage interest” almost certainly includes hidden fees or a balloon payment.
A borrower who cannot demonstrate a concrete plan to repay the capital at the end of the term — sale, savings, or other funds — will be turned away by most UK lenders. Even with a plan, the lender may still decline if the equity is too thin.
The pattern: Approval hinges on a credible repayment strategy and at least 25% equity — missing either shuts the door.
Is an interest-only loan a good idea?
What are the benefits of having an interest-only loan?
- Lower monthly payments during the interest-only period, freeing cash for investments or higher-priority spending.
- Can be useful for property investors who expect the property value to rise and plan to sell before the capital becomes due.
- Borrowers with a reliable lump sum (e.g., from a bonus, inheritance, or pension) can keep monthly costs low in the meantime.
The MoneyHelper (government-backed free money guidance) notes that an interest-only mortgage can work well if you have a clear, low-risk plan to repay the capital.
What are two disadvantages of an interest-only loan?
- No equity building – You own a smaller share of the property each year. If house prices fall, you could end up in negative equity.
- Large balance due at the end – The full loan amount must be repaid in one go. If your repayment strategy fails (investment returns fall short or the property doesn’t sell), you could be forced to sell at a loss or remortgage under worse terms.
Why would you pay interest-only on a loan?
Borrowers choose interest-only when they expect a future lump sum — inheritance, sale of another asset, or investment growth — and need the lowest possible monthly outlay in the meantime. Others use a temporary switch (the type HSBC UK calculates) to ride out a short-term income dip, like a job change or maternity leave.
The trade-off: you gain flexibility today but bet everything on the future payoff. If the bet fails, the lender still wants the full £200,000 back.
The catch: An interest-only borrower gains short-term cash relief but shoulders the risk that market timing, investment returns, or personal circumstances may not cooperate when the capital falls due.
What’s better, interest-only or repayment?
Monthly cost comparison: interest-only vs repayment
For the same loan amount and interest rate, an interest-only payment is always lower than a repayment payment because you are not repaying the capital.
The table below highlights the monthly saving — and the hidden long-term price.
| Loan details | Interest-only monthly | Repayment monthly | Difference |
|---|---|---|---|
| £200,000 at 5% over 25 years | £833.33 | Approx £1,169.86 | £336.53 cheaper per month |
| £200,000 at 3% over 25 years | £500.00 | Approx £950.00 (Charles Cameron & Associates) | £450 cheaper per month |
MoneySavingExpert’s calculator supports both repayment and interest-only mortgages, letting you see the trade-off instantly.
Long-term cost and equity comparison
Over a 25-year term, the interest-only borrower on a £200,000 loan at 5% will have paid £250,000 in interest and still owe £200,000. The repayment borrower at the same rate will have paid roughly £350,958 in total and own the house outright. The difference in total cost: £100,958 — plus owning the asset versus not.
Which option fits different borrower profiles?
- Owner-occupier who wants to own the home outright: repayment is the clear winner unless you have a guaranteed capital lump sum.
- Buy-to-let investor: interest-only keeps costs low, relying on property appreciation and future sale for profit.
- Borrower needing temporary relief: a short interest-only switch (e.g., 6 months) can help manage a crisis without long-term consequences — but only if you return to repayment quickly.
A repayment borrower who chooses interest-only to save £336 a month must invest that £336 somewhere that beats the mortgage rate — every month, for 25 years — just to break even. Most UK savings accounts currently pay less than 5%, meaning the interest-only borrower almost certainly loses wealth compared to the repayment borrower.
The verdict: For most owner-occupiers without a guaranteed lump sum, a repayment mortgage builds equity and avoids the balloon-payment risk that defines interest-only borrowing.
Pros and cons of interest-only loans
Upsides
- Lower monthly payments improve cash flow in the short term
- Allows borrowers to invest the difference elsewhere (property, stocks, business)
- Useful for temporary income gaps when a repayment strategy is solid
- Can make a higher-value property affordable for investors with a planned exit
Downsides
- No equity is built — house price drops can leave you in negative equity
- Full loan balance must be repaid at the end of the term
- Stricter approval criteria; fewer lenders willing to offer it
- Interest rate rises hit the monthly payment directly without the cushion of a reducing balance
How to use an interest-only loan calculator in 3 steps
- Enter your loan amount – the total you plan to borrow (e.g., £200,000).
- Enter the annual interest rate – as a percentage (e.g., 5%). Use the current rate you’ve been offered or a typical UK market rate from a site like MoneySavingExpert.
- Read the result – the calculator shows your monthly interest-only payment. To see the total interest over the term, multiply that monthly figure by the number of months (e.g., £833.33 × 300 = £250,000).
The MoneyHelper calculator and the Bank of England borrowing calculator both include fields for interest-only loans and display monthly interest paid separately.
Interest-only mortgage timeline
- Application: Borrower submits income, credit, valuation, and repayment strategy details; lender checks affordability using FCA rules.
- During interest-only period: Borrower pays only the monthly interest; the original loan amount remains outstanding. Any overpayments reduce the balance.
- End of interest-only period: Full outstanding capital is due. Options: refinance with a new product, sell the property, or use a planned lump sum.
- After the term: If the capital is not repaid, the lender can repossess. The borrower may negotiate a new mortgage, but terms may be worse.
What we know and what remains unclear
Confirmed facts
- A monthly interest-only payment is the loan amount multiplied by the annual rate, divided by 12.
- Interest-only payments do not reduce the loan principal.
- At the end of an interest-only mortgage, the outstanding capital must be repaid.
- UK regulated lenders must assess whether interest-only borrowers have a credible repayment strategy (FCA).
What’s unclear
- Which UK lenders currently offer interest-only mortgages and what minimum equity they require.
- How a specific lender treats investment performance when assessing a repayment strategy.
- Whether an interest-only loan is a good idea — it depends on the borrower’s cash flow, investment returns, and risk tolerance.
What the experts say
“A credible repayment strategy is not a ‘nice to have’ – it is a regulatory requirement. If you cannot show a realistic plan to repay the capital, an interest-only mortgage simply won’t be approved.”
“Switching to interest-only for six months could reduce your monthly mortgage payment substantially – but the overall cost of your mortgage will go up because you stop paying off the capital during that time.”
“The calculator gives you a ballpark figure. It uses the amount you borrow and the total interest paid, divided by the number of months – a simple way to see the difference between interest-only and repayment.”
Summary
An interest-only loan calculator shows a low monthly number — but that number is a trap if you have no plan for the balloon. For a UK borrower with no reliable lump sum and no investment portfolio, the decision is clear: choose a repayment mortgage, or overpay the interest-only as though it were a repayment mortgage. Otherwise you risk owning nothing while still owing everything.
respectmortgages.co.uk, fairmortgages.co.uk, calcforlife.com
Frequently asked questions
What inputs does an interest-only loan calculator need?
Most calculators ask for the loan amount and annual interest rate. Some also request the loan term (years) to show total interest over the period. No term is needed for the monthly payment calculation itself.
Can I make overpayments on an interest-only mortgage?
Yes, most UK lenders allow overpayments up to a certain limit each year without penalty. Overpayments reduce the outstanding balance, which means you will have less capital to repay at the end. Check your lender’s terms — some cap overpayments at 10% of the balance per year.
What happens if my interest-only mortgage reaches the end of the term?
You must repay the full outstanding capital. If you cannot, the lender may repossess the property. Options include remortgaging to a repayment product, selling the property, or using a planned lump sum. The FCA requires lenders to contact borrowers before the term ends to discuss repayment.
How does a temporary interest-only payment switch work?
Some lenders, like HSBC UK, allow existing borrowers to switch to interest-only for a short period (often 6–12 months) during a financial hardship. The loan still accrues interest, and the full capital remains due at the end of the original term. After the switch, payments revert to repayment.
Why do lenders ask about a repayment strategy for interest-only mortgages?
The Financial Conduct Authority made it mandatory in 2014. Lenders must check that you have a realistic plan to repay the capital at the end of the term — such as selling the property, using savings, or a pension lump sum. Without a credible strategy, the mortgage will be declined.
Do interest-only payments reduce the amount I owe?
No. Each month you pay only the interest on the loan. The principal (the amount you borrowed) stays the same. To reduce the principal, you must make overpayments in addition to the interest payment.
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